The processing of ultra high molecular weight polyethylene (UHMWPE), i.e. polyethylene having a molecular weight in excess of about 2 million, is known in the polymer arts to be extremely difficult. Products made from such materials are, however, very strong, tough and durable.
In the following series of U.S. Patents filed by Kobayashi et al and assigned to Nippon Oil Co., Ltd. a number of inventions related to the fabrication of fibers and films of polyolefins generally and UHMWPE specifically, are described: U.S. Pat. Nos. 4,996,011, 5,002,714, 5,091,133, 5,106,555, 5,200,129, and 5,578,373. The processes described in these patents generally describe the continuous production of high strength and high modulus polyolefin films by feeding polyolefin powder between a combination of endless belts disposed in an up and down opposing relationship, compression molding the polyolefin powder at a temperature below its melting point between the endless belts and then rolling and stretching the resultant compression molded polyolefin into an oriented film. As compression molded, the sheet is relatively friable thus requiring the subsequent calendering or drawing operations to provide an oriented film that exhibits very good strength and durability properties. In fact, the strength of such materials produced by these processes can be 3 times that of steel on a weight basis and they exhibit very low creep.
Enhanced processes for the production of such materials have also been described in the following U.S. patents and patent applications: U.S. Pat. No. 7,348,053 and U.S. patent application Ser. No. 11/217,279 filed Sep. 1, 2005.
A common element of all of these prior art processes is that they require compaction of an UHMWPE powder as the initial step in the production process. Until now, it has been the thinking of the UHMWPE manufacturing community that such powder compaction was necessary in order to place the material in a form that it could be subsequently rolled and drawn as described in the referenced prior art. Stated differently, it has been the thinking that in order to produce the product in a process involving the subsequent rolling and drawing steps to obtain the orientation required for the production of ballistically useful UHMWPE, the powder had to first be placed in the form of a sheet that demonstrated sufficient tenacity to be successfully processed in such subsequent rolling and drawing processes. In the prior art, such a form was obtained by compacting the powder into a relatively friable sheet that could be introduced into the rolling operation for subsequent processing.
The performance of this compaction process step, particularly in the production of UHMWPE sheets wider than 1-2 inches in width, requires the use of relatively massive, quite complex and very expensive equipment (measured in the millions of dollars for installed such equipment). Such equipment thus requires high levels of capital expenditures for installation and due to its complexity ongoing high operating and maintenance expenses. Additionally, compaction, as practiced in the prior art requires that the polymer be evenly distributed across and down-web (in the machine direction). Small variations in distribution (unequal mass) create defects in the sheet when it is subsequently calendered. Calendering requires that the mass entering the nip be substantially equal across the gap opening. If a streak or longitudinal area of the compacted sheet is low in polymer mass, it creates low density material in that area when it exits the calender. This results in a weak area in the film that can break during subsequent drawing, or leaves a weak place in the finished product. By introducing the polymer at the calender, the material can redistribute to a small degree and the resulting sheet has a much more consistent density. Even and consistent polymer distribution is one of the largest issues with any process that includes compaction.
U.S. Pat. No. 4,436,682 to Knopp, issued Mar. 13, 1984 describes a process for compacting polymer powders into fully dense products. According to this patent, a polymer powder is fed from a hopper into the nip between two rolls, compacted therein at a temperature below the melting point of the polymer powder and withdrawn from the nip under tension to form a “fully dense” polymer sheet. According to Knopp, when his process is applied to an UHMWPE powder, the resulting sheet has a density of about 0.82 g/cc which he designates as “substantially fully dense”. It is well known that the density of UHMWPE is on the order of above 0.945 g/cc. Hence, the product of Knopp's process is hardly “fully dense” and is unsuited to further processing by calendering or drawing, since it will tear or break when subjected to such processes. Additionally, the temperature requirements recited by Knopp are such that subjecting, for example UHMWPE, to such temperatures during rolling to form the coherent sheet would destroy the properties of the UHMWPE making it unsuitable for subsequent calendering or for the application of the produced sheet to ballistic or other high impact applications.
It would thus be of great benefit to the producer of such UHMWPE materials, particularly in widths greater than a couple of inches, if a much simpler, smaller and less expensive first process step could be substituted for the powder compaction step, without negatively affecting the either the product thus produced or significantly affecting the kinetics of the process, i.e. it did not, for example, slow production to an uneconomical rate.